Standard configurations of rotor blades typically used in aircraft rotary propulsion systems that allow variable pitch operation usually include a root attachment mechanism such as a ball/roller bearing and/or flex member, both of which allow pitch change of the blade with relatively low friction between components. To impart the necessary structural integrity to such mechanisms (e.g., to accommodate the substantial centrifugal forces exerted on the mechanisms during operation), they are often fabricated in such a way so as to be extremely heavy. A further complication is that substantial centrifugal loads on the plate-like structures of the blades themselves also produce significant twisting or turning forces that pitch control systems must overcome. These forces tend to turn the blade towards an undesirable flat pitch position. In the event that a malfunction of the pitch control system occurs, the forces acting on the blades could turn the blades to the flat pitch position, reducing rotor rotational resistance, thereby resulting in rotor overspeed conditions and potential blade loss.
As stated above, the turning force that acts on a rotor blade during its normal operation is substantial. In propulsion technology as it applies to fans and propellers, this force is referred to as the total twisting moment (TTM). The TTM is the net sum of three basic forces, viz., the centrifugal twisting moment (CTM), the aerodynamic twisting moment (ATM), and the frictional twisting moment (FTM). The CTM, which is typically the most substantial of the forces, originates from a non-symmetrical mass distribution of an airfoil of a rotor blade about a pitch change axis of the airfoil. In other words, in an oblong airfoil having a non-circular, non-symmetrical cross section, the mass about the pitch change axis is not evenly distributed, and centrifugal forces originating from the rotor's axis of revolution and acting on elements of the airfoil cause inertial twisting forces. The ATM is caused when the effective center of pressure on each section of an airfoil of a rotor blade is forward or aft of the pitch change axis. The FTM resists turning motion and develops in retention bearings that support the rotor blade due to high centrifugal loads acting on the bearings. In the operation of a rotor blade in which all three forces are taken into account, the CTM acts to turn a rotor blade toward low pitch, but because the aerodynamic center of pressure of an airfoil is usually forward of the pitch change axis, the ATM opposes and counters the CTM to turn the rotor blade toward an increased blade pitch. The FTM, which is caused by friction, opposes blade pitch change in either direction.
The forces of the pitch control system required to overcome the forces acting on the rotor blade during its operation can be appreciable. With TTM being dominated by CTM, the pitch control system of a typical rotor blade device exerts a torsional load in the direction of increased pitch to hold the blade pitch constant. The system must also exert an additional force to overcome the FTM in order to increase the blade pitch. However, if there is a malfunction and/or loss of control of the pitch control system (e.g., due to loss of engine power), a rotor blade will naturally turn toward lower pitch. Because low pitch results in less rotational resistance for the engine, the situation can result in an undesirable overspeed of the rotor and engine. In extreme conditions in variable pitch systems with no low pitch stop, the TTM can turn the blades to low pitch, and rotor thrust can suddenly switch to a high drag force that can cause possible loss of aircraft control and/or result in rotor overspeed. Rotor overspeed is more likely if the rotor is driven by a turbine engine rather than a piston engine, especially if that segment of the turbine that powers the rotor is separate from other turbine components. This turbine is referred to as a “free” turbine (i.e., there is no revolution limiting capability). In a single engine aircraft, increased drag can limit glide distance for an unplanned landing, while in a twin engine configuration, the asymmetric drag of one disabled propulsor can hinder the ability of the pilot to control the aircraft.
To prevent undesirable pitch tendencies, counterweights have been added to the sides of rotor blades and at or proximate the root ends of the rotor blades. Such weights are typically of sufficient mass to create a net TTM that will be able to overcome all inherent rotor blade turning forces and drive the rotor blades toward higher pitch (or at least maintain the pitch setting to prevent movement toward lower pitch). These weights have also been known to be substantial in mass, thereby adding unsprung weight to the rotor blades and further loading the bearings associated with the rotor hub and blade retention mechanisms. Also, these weights often have associated retention mechanisms or other devices that may be prone to failure under normal operating conditions due to the mechanical stresses encountered. If a failure is experienced, the high energy of the released mass may result in impact damage as well as high rotor unbalance conditions. Other pitch control systems typically employ auxiliary electric pumps that provide backup pressure for a hydraulic system, linear ACME thread harmonic drives, and/or latching devices that hold position. In at least some of these systems, if the pitch of the rotor blades is maintained in a less than optimum position for gliding (in an aircraft having a single engine configuration) or for compromised operation (in an aircraft having a twin engine configuration), increased drag forces may be generated which inhibit the ability of an operator to properly manage the system.
Based on the foregoing, what is needed is a device for efficiently and controllably varying the pitch of a rotor blade in an aircraft propulsion device. Also, what is needed is a rotor blade for an aircraft propulsion device that is capable of being efficiently and controllably varied.